Sudoku

Sudoku (originally called Number Place) is a logic-based, combinatorial number-placement puzzle game. The objective is to fill a 9x9 grid with digits so that each column, each row, and each of the nine 3x3 sub-grids that compose the grid contains all of the digits from 1 to 9.

It requires logical thinking ability, many people believe it is a good way to exercise their brains.